Examinando por Autor "Allard, F."

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    Temperature constraints on the coldest brown dwarf known: WISE 0855-0714
    (EDP Sciences, 2014-10) Beamín, J.C.; Ivanov, V.D.; Bayo, A.; Mužić, K.; Boffin, H.M.J.; Allard, F.; Homeier, D.; Minniti, D.; Gromadzki, M.; Kurtev, R.; Lodieu, N.; Martin, E.L.; Mendez, R.A.
    Context. Nearby isolated planetary mass objects are beginning to be discovered, but their individual properties are poorly constrained because their low surface temperatures and strong molecular self-absorption make them extremely faint. Aims. We aimed to detect the near-infrared emission of the coldest brown dwarf (BD) found so far, WISE0855-0714, located ~2.2 pc away, and to improve its temperature estimate (Teff = 225−260 K) from a comparison with state-of-the-art models of BD atmospheres. Methods. We observed the field containing WISE0855-0714 with HAWK-I at the VLT in the Y band. For BDs with Teff< 500 K theoretical models predict strong signal (or rather less molecular absorption) in this band. Results. WISE0855-0714 was not detected in our Y-band images, thus placing an upper limit on its brightness to Y> 24.4 mag at 3σ level, leading to Y − [ 4.5 ] > 10.5. Combining this limit with previous detections and upper limits at other wavelengths, WISE0855-0714 is confirmed as the reddest BD detected, further supporting its status as the coldest known brown dwarf. We applied spectral energy distribution fitting with collections of models from two independent groups for extremely cool BD atmospheres leading to an effective temperature of Teff< 250 K,
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    VLT/SPHERE survey for exoplanets around young early-type stars, including systems with multi-belt architectures
    (EDP Sciences, 2020-07) Lombart, M.; Chauvin, G.; Rojo, P.; Lagadec, E.; Delorme, P.; Beust, H.; Bonnefoy, M.; Galicher, R.; Gratton, R.; Mesa, D.; Bonavita, M.; Allard, F.; Bayo, A.; Boccaletti, A.; Desidera, S.; Girard, J.; Jenkins, J.S.; Klahr, H.; Laibe, G.; Lagrange, A.-M.; Lazzoni, C.; Marleau, G.-D.; Minniti, D.; Mordasini, C.
    Context. Dusty debris disks around pre- and main-sequence stars are potential signposts for the existence of planetesimals and exoplanets. Giant planet formation is therefore expected to play a key role in the evolution of the disk. This is indirectly confirmed by extant submillimeter near-infrared images of young protoplanetary and cool dusty debris disks around main-sequence stars that usually show substantial spatial structures. With two decades of direct imaging of exoplanets already studied, it is striking to note that a majority of recent discoveries of imaged giant planets have been obtained around young early-type stars hosting a circumstellar disk. Aims. Our aim was to create a direct imaging program designed to maximize our chances of giant planet discovery and target 22 young early-type stars. About half of them show indications of multi-belt architectures. Methods. Using the IRDIS dual-band imager and the IFS integral field spectrograph of SPHERE to acquire high-constrast coronagraphic differential near-infrared images, we conducted a systematic search in the close environment of these young, dusty, and early-type stars. We used a combination of angular and spectral differential imaging to reach the best detection performances down to the planetary mass regime. Results. We confirm that companions detected around HIP 34276, HIP 101800, and HIP 117452 are stationary background sources and binary companions. The companion candidates around HIP 8832, HIP 16095, and HIP 95619 are determined as background contaminations. Regarding the stars for which we infer the presence of debris belts, a theoretical minimum mass for planets required to clear the debris gaps can be calculated. The dynamical mass limit is at least 0.1 MJ and can exceed 1 MJ. Direct imaging data is typically sensitive to planets down to ~3.6 MJ at 1′′, and 1.7 MJ in the best case. These two limits tightly constrain the possible planetary systems present around each target. These systems will be probably detectable with the next generation of planet imagers. © M. Lombart et al. 2020.